[New post] The Jurassic Coastline From the Sea – A New Perspective
devongeography posted: " While visiting West Bay recently, I had the opportunity to take a boat trip toview the local coastline from a slightly different perspective. I was lucky to be there on a lovely summer day when the sea was mirror-calm to enjoy a 'Golden Hour' cruise o" Devon Geography
While visiting West Bay recently, I had the opportunity to take a boat trip toview the local coastline from a slightly different perspective. I was lucky to be there on a lovely summer day when the sea was mirror-calm to enjoy a 'Golden Hour' cruise offered by the Lyme Bay Rib Charter company, who operate from the inner harbour.
The boat trip provided a great chance to view part of the 95 mile long Jurassic Coast, a World Heritage Site, and the jewel in the crown of England's south coast. This wonderful piece of coastline displays a continuous sequence of rock formations spanning 185 million years - from the Triassic rocks to the west (252 - 201 million years ago), through Jurassic rocks (201 - 145 million), and on to the Cretacious rocks (145 - 66 million) further east.
Before I was able to get close to the coastal geology, leaving the quay allowed for some close-up views of the sea defences that have been installed to improve the harbour at West Bay.
Slipway and rock armour protection , West Bay harbour. Image - Paul Berry
West Bay harbour stands at the mouth of the river Brit and was constructed in 1722. Over time, it has been beset by nature's problems, enduring being silted up, blocked by shingle, and damaged by storms. A series of repairs and enlargements have been required, and the harbour has been moved twice from its original position one mile inland, firstly to the coast by East Cliff where the river once met the sea, and then 270 metres to the west to its present location.
West Bay once exported ropes and nets from nearby Bridport, while importing gravel, timber and coal. Shipbuilding here dates from the days of Alfred the Great in the ninth century, and there were large shipbuilding yards to the west of the harbour, turning out cutters, schooners, barques and fishing smacks. At one time, 300 workers were employed here, and by 1830, over 500 vessels used West Bay harbour each year.
As I exited the harbour, a view opened up to the cliffs to the north that would be encountered in the second part of the voyage. Beyond the piers and sea wall could be seen the rising heights of West Cliff, and beyond that, the peaks of Thorncombe Beacon, Doghouse Hill and Golden Cap.
East Pier, with West Cliff, Thorncombe beacon, Doghouse Hill, and Golden Cap in the distance. Image - Paul Berry
Diagram - Paul Berry
From the harbour, the rib boat turned south east towards Burton Cliffs to start our exploration of the coastline. The cliffs here are mainly Bridport Sands, topped by thin layers of Inferior Oolite (a hard limestone) and Frome Clay (also known as Fullers Earth), and the area is prone to frequent rock falls.
Freshwater Beach, with the river Brid to the left. Image - ukcampsite.co.uk
Turning northwards, the rib headed back towards West Bay, passing the large holiday camp at Freshwater. Here, the river Bird runs through the shingle beach built high to protect the holiday camp from flooding in severe storms. This is actually the start point of Chesil Beach, the famous 18 mile long shingle barrier beach that extends southwards to the Isle of Portland.
Beyond Freshwater we come to the highlight of area, the famous 'Broadchurch' cliffs.
Diagram - Paul Berry
Panorama of East Cliff. West Bay is to the left, and Burton Cliff on the extreme right. Image - Paul Berry
East Cliff forms one of the most distinctive profiles along the entire Dorset coast. These golden cliffs of Bridport Sands, are rocks formed around 175 million years ago. The sandstone beds here are exposed here to a height of around 43 metres, and have an overall depth of 49 metres. Each metre of the Bridport Sands took 20,000 years to accumulate, meaning it would have taken 860,000 years for all the exposed cliff height to be created.
East Cliff. Image - Paul Berry
The Bridport Sands Formation is one of the major sandstones in the Jurassic sequence in Dorset, being laid down in the early Jurassic, in the Toarcian Age, between 182 and 174 million years ago. During the early Jurassic, the supercontinent of Pangea began to split apart and the world experienced a significant increase in plate tectonic movement, volcanic activity and mountain building. This was therefore a period of significant global change, as continental configurations, oceanographic patterns, and biological ecosystems were altered. As far as Dorset was concerned, during the early Jurassic there was a deep sea covering this area, with thick clay deposits being laid down. As the seas became shallower, rivers brought material from further north, and deposited sands at the front of a huge delta that began to grow southwards.
Yellow Bridport Sands of East Cliff. Image - Paul Berry
The rocks at East Cliff vary in colour from bright orange to earthy brown. This hides the fact that they are naturally a blue/grey colour, the oranges in the cliff being due to oxidation at surface of fine pyrite grains (fools gold) which changes it from a mineral with a metallic lustre to a dull iron ore called limonite. If you look at freshly broken piece of cliff you can usually see the original colour.
Horizontal bands of Bridport Sands. Note the narrowing of beds towards the top of the cliffs. Image - Paul Berry
More resistant beds jut out of the cliff face giving East Cliff a ridged surface. Image - Paul Berry
The sedimentary beds of East Cliff are clearly horizontal, much as they were when they were first deposited. They do however, display a ridged face, where layers of hard, carbonate cemented sands alternate with less well cemented sands. These layers weather and erode at different rates producing the corrugated surface. As calcium carbonate in sea water precipitates out, it acts as a cement for the sandstone, and the cliffs here offer a clear demonstration of the cyclical nature of environmental change. Calcium carbonate concentrations in the sea have varied over time, with storm conditions bringing more organic detritus (which uses calcium carbonate for hard parts) which increases the amount of 'cement' and creates the 'harder' rock layers. If you study the cliffs closely, you can see that the harder layers of sandstone are more closely spaced at top rather than at the bottom. Does this mean that stormy times became more frequent to create these layers, or that at that time, the supply of sand was decreased?
Joints in the cliffs produce recesses and buttresses. Image - Paul Berry
East Cliff is full of vertical joints that have opened up in the sandstone, many creating buttresses and recesses, some that have been deepened out into caves. At the top of the cliffs, some joints have been opened up and then infilled with the debris of rocks above. These splits in the rock surface create perfect nesting sites for seabirds.
The cliffs here experience rapid undercutting when the beach level is low, and wave cut notches do appear in places. Higher beach levels protect the cliffs to a degree, along with unsupported buttresses that have collapsed as rock falls, but it must be remembered that the prevailing south westerly winds here blow across a fetch of ocean that stretches for 3100 miles, so there is quite an onslaught delivered by marine erosion.
Recent landslip at East Cliff. Image - Paul Berry
Landslips have always been a significant factor in these cliffs, assisted by non-marine processes that help to create slipping and mass slumping with the clay sliding over the lower layers , a situation made worse by the numerous faults in the cliffs. A major rockfall occurred at East Cliff on 28th June, 2017, after a long dry spell was followed by heavy rainfall. The coastal footpath was cut off, and there was some loss of land from the clifftop golf course. It is impossible to offer any reliable predictions for events such as this, and the cliffs here could be stationary for 100 years before a sudden fall. However, it is disturbing to see so many visitors occupying positions on the beach directly below the cliffs, while others take their chances right on the very edge of the cliff tops.
Truncated 'hanging valley', location of the sixth hole on Bridport's golf course. The tee is on top of the far hillside, and the green at the bottom of the valley. Image - Paul Berry
A few hundred metres from West Bay, a clear dip in the sandstone cliffs can be identified. This is a small dry valley, inclined seawards, that resembles a dry chalk coombe. Here, in the cold phases of the Pleistocene ice age (including the last phase called the Devensian), the frozen sandstone floor became impermeable.and held up running melt water from warmer spells to enable fluvial erosion to cut a valley. Under these periglacial conditions (there was no actual ice cover this far south), 'head' or hill wash debris was transported from the valley slopes to the valley floor due to solifluction as the ground cycled through periods of freezing and melting. This is visible in places at the top of the cliffs as a reddish layer on the yellow Bridport Sands. The valley was eventually truncated by coastal erosion and landslips, altering the lay-out of hole six, the signature hole on the Bridport and West Dorset golf course.
Rock armour protecting the harbour entrance. Image - Paul Berry
Our voyage takes us to the harbour entrance once more, where we pass close to the thick barrier of rock armour (formed of Scandinavian larvkite rock) that protects the harbour piers. When the port of West Bay was moved to its present position, two piers were constructed to house the harbour, with the river Brit being diverted to run between them. In 2005, a new coastal defence scheme came into being, with the west pier replaced (and renamed the Jurassic Pier), the east pier rebuilt, a new slipway built, and the outer harbour area created. It meant the harbour could now be used on the 50% of days when southerly swell conditions occurred , which was previously not possible.
Sailing beyond the harbour, we headed towards West Cliff. On the way, we saw more of the coastal defence scheme, including a sea wall, more large protective boulders added to the foreshore, an extended promenade, and regraded cliffs to protect from landfalls.
Extended sea wall running from the harbour to West Cliff. Slopes have been regraded to prevent landslips. Image - Paul Berry
At West Cliff (also known as Watton Cliff), the coastal rocks changes from the orange beds of the Bridport Sands to grey cliffs of slumped clay. The geology now is primarily Frome Clay (or Fullers Earth) topped by the Middle Jurassic Forest Marble, part of the great oolitic limestone series. In between these two layers is a distinctive lighter coloured bed about 30 cm thick, made up of crushed shells (known as the Boueti Beds). The name 'Fullers Earth' comes from its absorbent properties that led to its use in fulling or degreasing of wool - although the local Fullers Earth happens not to be suitable for this process. Forest marble is not a true marble, but rather a shelly limestone that takes a good polish. It is widely used as a building stone and for paving, and is commonly seen in the town of Bridport.
Grey clay slopes of West Cliff. Image - Paul Berry
Just north of West Cliff is the area known as Fault Corner. A number of faults on this stretch of coastline complicate the local geology, making it difficult even for experts to interpret the strata.
Fault Corner. Image - Paul Berry
At Eype Mouth, a stream has cut a steep-sided valley. One of the most important faults in the region is found here. The rock layers on the eastern side of the fault have dropped down around 200 metres, bringing younger grey clays right next to much older yellow sandstone.
Eype Mouth. Image - Paul Berry
Passing Hope Corner, we could pick out the tall hill of Thorncombe Beacon. This was a crucial signalling station for Lyme bay in the days of the threats from the Spanish Armada, providing easier access than the higher point of Golden Cap a little further along the coast. Apart from very top which is of Cretaceous age, the bulk of Thorncombe Beacon is composed of Bridport Sands. A boulder arc at the foot of the cliffs marks the extent of former land slips.
Cliffline at Hope Corner, approaching Thorncombe Beacon. Image – Paul Berry
Hope Corner. Image – Paul Berry
The lesser peak of Doghouse Hill comes next, the site of Dorset's oldest human settlement that has revealed a Bronze Age stone hearth, fire pits, and pottery.
Thorncombe Beacon. Image - Paul Berry
The small hamlet of Seatown occupies another steep-sided valley. Once home to between 30 and 40 mackerel fishermen, the settlement is now is a small tourism centre. Back in the day, the residents here were undoubtedly also associated with smuggling, and Seatown was the haunt of the notorious nineteenth century 'Chideock Gang' who operated their boats from here.
Seatown. Image - Paul Berry
Our next landmark is Golden Cap, at 191 metres, the highest cliff in southern England. The capping of Upper Greensand rock which sits on top of Jurassic clays gives this feature its name, catching the light even on rather dull cloudy days.
Golden Cap. Image - Paul Berry
Golden Cap has its own distinctive profile, created by numerous past landslips. The Cretaceous Upper Greensand rocks that cap the cliffs are permeable, but there are impermeable rocks below that prevent the downward movement of water. This leads to the rocks immediately below the Greensand becoming unstable and no longer able to support the rocks above, and this causes the collapse of the Greensand in huge landslips.
So if there is Cretaceous strata lying on top of Lower Jurassic Clays, where is the rest of the Jurassic, and for that matter the Lower Cretaceous rocks? Thousands of metres of strata are missing here at Golden Cap!
Golden Cap. Image - Paul Berry
These absent rocks have been eroded away, making Golden Cap one of a number of places on the Jurassic Coast where the 'Great Unconformity' is on show. Sometime during the Lower Cretaceous, earth movements caused this part of the world to be uplifted. It became land and any Upper Jurassic and Lower Cretaceous rocks that had been deposited were eroded. The sedimentary strata were also tilted by the earth movements , so when the sea returned during the Cretaceous period, subsequent sedimentary were laid down at an angle to these older rocks. Further west in Devon, Cretaceous strata are directly on top of Triassic layers, with the entire Jurassic having been eroded away.
Golden Cap. Image - Paul Berry
Before we reach Charmouth, we pass Stonebarrow Hill. with the mudslides of Cain's Folly below. The upper slopes of Stonebarrow are scarred by landslips , and in 1942, a radar station was displaced by a huge fall, sliding down to Cain's Folly below. The crew are said to have stepped out unharmed as the concrete building came to a halt. Successive mudslides at Cain's Folly have delivered debris to the beach over the steep cliffs on seaward side.
Cain's Folly. Image – Paul Berry
Cains's Folly below Stonebarrow Hill. Image – Paul Berry
Charmouth soon appears, and the excellent Visitor Centre close to the beach can be clearly seen - a great place to find out about the rich fossil finds that can be discovered in this area.
Charmouth. Visitor Centre is large building to the left of the beach. Image - Paul Berry
Directly to the west of Charmouth beach is one of the best fossil sites in the country - the unstable cliffs of Black Ven. Belemnite, ammonites, crinoids, and gryphea fossils can all be found here in the clays and marls. Black Ven is an active landslide complex, and there have been numerous repeat performances of cliff collapses here over time. It is considered to be the largest mudslide in Europe, and the constantly shifting landscape means no vegetation has been able to get a hold on the ground.
Black Ven. Image - Paul Berry
Ever-shifting slips repeat a little further along the coast as we approach Lyme Regis. here, at The Spittles, the coast road was carried away by a series of landslips which started in 1920 and continued for the rest of the century.
The Spittles. Image – Paul Berry
The Spittles. Image – Paul Berry
Church Cliffs pic 34 34 35 50
XS LR-GC
Directly to the east of Lyme Regis, Church Cliffs have been protected by a lengthy sea wall. As we sail on past the town, more sea defences can be spotted, including sea walls, rock armour, and rock groynes.
Sea Wall protecting Church Cliffs. Lyme Regis is on the left. Image - Paul Berr
Lyme Regis. Image - Paul Berry
Rock armour protection at the end of the Cobb. Image - Paul Berry
Our voyage ends at the Cobb, the giant stone pier that formed part of the medieval harbour of the town. We turn here, to head back to West Bay.
Safely back on dry land, there is just enough time for a quick Jurassic-themed refreshment before returning home.
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